Hi,
There is probably a header tank for the engine and a separate one for the heating system. It is very unlikely that the hot water from the Boatman is passing through the same coil in the calorifier as the water from the engine. As Matty has said it is likely to be caused by air in the system. Try bleeding the radiators and if that doesn't fix it, work out which coil in the calorifier is connected to the Boatman and bleed any air if you can by loosening the connections until anti freeze/water comes through. You might need to do a similar bleeding of any connection in the system you can find.
Most people do use automotive anti freeze in their heating systems. You will need to be careful which one you add, if you need to, since they are not all compatible with each other. Check the colour. If you need to top up, mix the antifreeze with water before you add it to the system.
Phil